The increase in theft and related crimes has created the need for more effective means of surveillance. In the past, surveillance systems attached an electromagnetic device to an item to be monitored and placed sensors at the boundaries of the area of interest (i.e., typically at the exits of a store). In the event of an unauthorized taking, the sensors would be triggered by the electromagnetic device and an alarm would sound in the monitored area. However, such devices failed to cover the area of interest surrounding the item and, thus, did nothing to inhibit the completion of the unauthorized action. For example, once an unauthorized person exited a store with a monitored item, the alarm would sound at the exit of the store. However, the person could escape, for instance, in a crowd unnoticed. Furthermore, if the electromagnetic device was removed or broken within the store, the item could be removed from the store without triggering the alarm at all. Moreover, other articles carried by customers had a tendency to trigger the sensors, thereby sounding the alarm unnecessarily.
To remedy these unwanted circumstances, subsequent systems have provided self-powered, releasably attachable alarm devices producing an alarm at the exact location of the unauthorized taking (i.e., anywhere in the store, not just exits). Such devices are triggered by placing an antenna within the alarm device and using radiated energy to signal the alarm device when an unauthorized taking is in progress. One problem facing these later systems has been the control of the radiated energy. These systems have used radio waves at various frequency levels to signal the alarm devices. However, these signals are very difficult to control in the environment in which they are intended to be used, e.g., a retail clothing store.
Therefore, later systems have used a plurality of signals to define the area to be monitored. The use of various signals requires the use of multiple transmitters which, in turn, requires that the display, checkout, fitting, and exit areas be provided with transmitters radiating different frequencies. Such complex systems not only detract from a user's flexibility in covering the monitored area, but also require more power to operate.
Such complex signal generation also forces the alarm device to be more complex in design to discern the type of energy being received at any given moment. Complex alarm devices not only use more power, but require more maintenance.
It is, therefore, desirable to provide a system which simply, but accurately, defines an area to be monitored so that an alarm may be triggered from the item itself.